Will a subwoofer add depth and clarity to my system, or just bass?

Plga, are you using the QC 500’s yet?  If so, you are not getting foundational bass response...you may be pleased with what you are hearing but 41hz @ -6db is not room shaking.  Stereophile even mentioned that a good sub would be beneficial.

Yes, Im using the Q Acoustics Concept 500 and I'm getting more bass shaking than I need.
In some reviews they mention that the bass is outstanding in this floorstanders and I couldnt agree more. It has everything, impact, dinamic, tone, etc. They have fantastic bass.
It is also true that I boosted the bass a little bit with Dirac, both the low and upper bass, along with the rest of the Digital Room Correction the sofware does.
Its incredible the bass you get with a nice pair of floorstanders and Room treatment (active and pasive).
I strongly believe you dont need more (for music) or, to have it, you have to spend a considerably amount of money, time and effort.
How much would it cost me to buy two subs at the level of my speakers??  at least 6 to 8k... and, how much time would I have to spend voicing them to the room? not even considering the space they need, that most of us don´t have. I should better spend that money on electronics as Im sure I would get much better results as I did.
Im sure with subs you can boost the bass more and have a some more impact and deepness, but for me it could sound unreal if you are not careful.
I repeat, IMHO, the musical experience is much more complex than just adding extremely low bass (under 40 Hz) and if you dont do it extremely right, Im sure you will mess up the sound of your speakers with the subs, because it happened to me and it was NOT subtle.
I repeat, if you want to get your system to sound as real music as possible, you have to address many different variables: standing waves, tone, soundstage, dinamics, detail, smooth highs, etc. If, and only if, you succeed to match the subs perfectly your speakers, IMHO, you will only get a part of the problem resolved.
And if you listen to music other than electronic (jazz, blues, rock, accoustic, etc) getting a deep bass wont give you much as many recordings in that kind of music DONT have it.
Again, just my opinion.

Glad your happy plga...better off not knowing sometimes anyway.  Saves money!

Hey, it is just my experience and YMMV.
For me its much more important, easier and may be cheaper to address issues like standing waves in bass frequencies, early reflections and reverberance in highs, jitter (if you listen to digital), speaket positioning and toe in, listening position, AC power conditioning, etc.

And yes!
Audio improvement its a neverending story!!! Its an addiction!!  :-)